1. Technical Field
This invention relates to a source voltage monitor for a photographic camera having a built-in battery commonly used to energize various components such as a film transporting motor, a lens barrel driving motor, a shutter blade driving motor and a strobe. The device is adapted to determine whether a source voltage is at a level sufficiently high to assure effective operation of release for exposure and to disable the release if the source voltage is determined to be insufficient.
2. Prior Art
Handy photographic cameras, arranged such that users having neither special skill nor special experience in photography may easily and reliably take a photograph, have become very popular. Such cameras are usually provided with various components, such as an automatic film transporter, an automatic variable focus feature, and a strobe. The automatic film transporter is operatively associated with a rear cover of the camera body and a built-in motor so that, upon closure of the rear cover after a film cartridge has been loaded into the camera, the motor is actuated to advance the film roll thus loaded until the first frame reaches a position ready for exposure when a first photograph is taken. Upon release, the motor is actuated again to advance the film roll by one frame so as to bring the next frame into alignment with the aperture and, as soon as all the frames have been exposed, the film roll is rewound into the cartridge. The automatic variable focus feature is generally classified as either a two focus change-over type, adapted to change-over the focal distance between normal and telephoto focal distances, and a continuous focus changing type, adapted to change the focal distance in a continuous fashion. Both the change-over type and the continuous changing type automatic focuses are achieved by axially moving the lens barrel, which holds the objective therein, selectively forward or backward through actuation of the lens barrel driving motor.
The strobe is automatically discharged, when it is determined on the basis of object luminance data derived from an automatic exposure meter that a current luminance on the object to be photographed is insufficient for effective photographing.
A color picture of a subject's face taken using the strobe flash light is sometimes disadvantageously accompanied with so-called red eye phenomenon, in which pupils of the subject's eyes are reproduced in red. To avoid this red eye phenomenon, some photographic cameras are equipped to preflash discharge a flash lamp to light up the face before the strobe is discharged. More specifically, pupils of the eyes close responsive to this preliminary flash, and then the picture is taken as the strobe is discharged.
In taking a photograph using a camera equipped with a strobe having such a preflash function, the user can select any one of three modes for the strobe, i.e., preflash mode in which a preflash discharge occurs prior to strobe discharge, low luminance automatic strobe discharge mode in which the strobe is automatically discharged in response to a signal representing a luminance on the object lower than a predetermined level, and a forcible strobe discharge mode in which the strobe is forcibly discharged, regardless of the luminance level.
To reduce the weight and size of the camera, a common built-in battery is utilized as a power source serving to energize the previously mentioned various components, such as the film transporting motor, the lens barrel driving motor, the shutter blade driving motor and the strobe.
Since these popular cameras equipped with the above-mentioned various components must be easy to use and reliable even when the person taking the photograph has little knowledge and experience in photography, every release for exposure must be done with an appropriate, automatically adjusted lens opening and shutter speed. Since the shutter blades are opened and closed by the associated motor, it is important to disable operation of the release when the source voltage is determined to be insufficient to actuate the shutter blade driving motor. To this end, the camera of this type is equipped with a source voltage monitor adapted to determine, prior to operation of release for exposure, whether the source voltage is at a level sufficiently high for effective release.
The acceptable lower limit of the source voltage for effective release for exposure depends upon the mode of photographing. Specifically, in the low luminance automatic discharge mode, the lower limit corresponds to the minimum voltage (V.sub.0) necessary to actuate the shutter blade driving motor if the automatic exposure meter indicates that an object to be photographed has a luminance sufficient for effective exposure, since the strobe is not discharged. If the luminance is low and thus the strobe must be discharged, the lower limit of the voltage demand will correspond to the minimum voltage (V.sub.1) necessary to actuate the shutter blade driving motor and to discharge the strobe. Also in the forcible strobe discharge mode, the lower limit of the voltage demand corresponds to the minimum voltage (V.sub.1) necessary to actuate the shutter blade driving motor and to discharge the strobe. In the preflash mode, the lower limit of voltage demand is equal to the minimum voltage (V.sub.2) necessary to discharge the flash lamp, the strobe, and the shutter blade driving motor.
With the source voltage monitor of the prior art, the reference voltage used for comparison depends on the mode of photographing. More specifically, the voltage V.sub.1 is used as the reference voltage in the low luminance automatic strobe discharge mode, as well as in the forcible strobe discharge mode, and the voltage V.sub.2 is used as the reference voltage in the preflash mode in order to evaluate the source voltage.
However, such source voltage monitor of the prior art, disadvantageously requires relatively complicated monitor processing, since the reference voltage must be set for each mode of photographing and the processing procedure must be changed depending on the mode of photographing.
In addition, separate circuits must be provided to obtain the respective reference voltages, so the circuit arrangement is also complicated and the number of parts correspondingly increases.